The researchers studied how 1,687 people reacted to rigorous exercise. They found that 10 percent of the participants got worse, based on one or more measures of heart disease, including blood pressure and levels of insulin and HDL cholesterol. In fact, 7 percent got worse on at least two measures. Those results are published in PLoS One.

The weird thing is, nobody knows why. Claude Bouchard, one of the researchers and professor of genetics and nutrition at the Pennington Biomedical Research Center, could only muster that the finding "is bizarre" when quizzed by the New York Times.

In fact, there's a bit of a problem with studies about the benefits of exercise on health. While many have shown in the past that exercise can have an affect on measures of health—usually in a way that we'd think of as positive—very few of them carry through the analysis to see if participants suffer fewer heart attacks or live longer lives.

It might shock you to hear that most of the guidelines that recommend how much exercise you should take are based largely on such studies. While they've shown that exercise causes some positive change to some biomaker that in the short-term makes you medically "healthier"—they don't necessarily guarantee longer life.

Is that enough to stop exercising? No, absolutely not. This one study is interesting in so much as it may be the case that for a small number of people not exercising is better for them. But it's not a general rule.

In fact, even the authors of the study agree. As they explain, no medical intervention works for 100 percent of people. If you think of exercise as a medical intervention, then it's bound not work for a tiny percentage of the population—but for the rest of us, it can make a huge difference. [PLoS One via New York Times]